Asthma is a chronic respiratory disease characterized by inflammation, airway hyperresponsiveness (AHR) and mucosal edema, which together lead to episodic bronchoconstriction and obstruction of the airways. The effectiveness of current anti-asthmatic treatments is unsatisfactory, and asthma remains an unresolved global issue.
The tone of the musculature of the airways is determined by a delicate balance between activation of the pro-contractile and pro-relaxation signaling pathways in smooth muscle cells. Contraction is primarily triggered by acetylcholine, the main parasympathetic neurotransmitter in the airways, which activates M3 muscarinic receptors, leading to mobilization of intracellular and extracellular calcium (Ca2+). Conversely, relaxation of the airways is achieved by catecholamine-mediated activation of β2-adrenergic receptors (β2-AR), which promote the production of cyclic AMP (cAMP) and the consequent modulation of key effectors of Ca2+ homeostasis. According to the pro-relaxing action of cAMP, agonists of β2-ARs provide symptomatic relief of bronchospasms in patients with asthma. However, their effectiveness is limited in time, mainly due to the desensitization of β2-ARs that occurs after repeated exposure to agonists. Similarly, inhibiting cAMP degradation by inhibitors of phosphodiesterase 4 (PDE4), the main enzymes responsible for cAMP hydrolysis in the airways, has been clinically tested, but displays unacceptable side effects, such as vomiting, nausea, diarrhea and weight loss, due to non-selective inhibition of PDE4 in the central nervous system.
Therefore, identifying new enzymes that regulate cAMP homeostasis, as well as novel strategies for manipulating the β2-AR/cAMP signal transduction pathway in smooth muscle cells is desirable for treating respiratory diseases. Moreover, the same approach could also be used for therapeutic purposes in other pathological contexts, such as cystic fibrosis, where it is necessary to increase the levels of cAMP in the epithelial cells of the airways.
In the respiratory epithelium, the production of cAMP downstream of β2-ARs is necessary to ensure the opening of the cAMP-dependent chloride channel (cystic fibrosis transmembrane conductance regulator, CFTR). Mutations in the gene that encodes for this protein are the main cause of cystic fibrosis (CF). Among these, deletion of phenylalanine 508 (ΔF508) constitutes the most common alteration in CF patients and leads to defects in both membrane expression and opening of the channel. A number of CFTR corrector and potentiator drugs, which rescue the membrane expression and the cAMP-mediated opening of the channel, respectively, have been developed, but their effectiveness is unsatisfactory. In particular, CFTR potentiators require high concentrations of intracellular cAMP to be effective. Therefore, drugs that are able to stimulate cAMP levels may constitute novel strategies to increase the effectiveness of currently available treatments, or to directly correct functional defects of CFTR in CF.
Previous studies have shown that phosphoinositide 3-kinase γ (PI3Kγ) controls the compartmentalization of cAMP downstream of β2-AR. In cardiomyocytes, PI3Kγ acts as an anchor protein (AKAP) (1), which binds protein kinase A (PKA) to several isoforms of PDE3 and PDE4. PI3Kγ-associated PKA in turn phosphorylates and promotes the activation of the PDEs and the consequent reduction of cAMP downstream of β2-ARs, ultimately limiting the arrhythmogenic release of Ca2+ (2). Although several inhibitors of the kinase activity of PI3Kγ have been developed, there are currently no methods for selectively interfering with the adaptor or anchor protein activity of PI3Kγ.